Conventionally, nitride semiconductor blue light-emitting elements using a GaN based nitride semiconductor for an active layer have been widely used. However, nitride semiconductor ultraviolet light-emitting elements using an AlGaN based nitride semiconductor having a shorter emission wavelength than the GaN based nitride semiconductor for an active layer still have not been widely used.
This is because AlGaN based nitride semiconductors composed of mixed crystals mainly of AlN and GaN have a special characteristic that bonding strength of Al and N in AlN is much larger than bonding strength of Ga and N in GaN, so that high quality crystals are difficult to grow. In particular, even if the growth method of a GaN based nitride semiconductor, which has already been established, is employed as a growth method of an AlGaN based nitride semiconductor, it is impossible to form crystals that have quality as high as GaN, which is problematic.
To handle such a problem, various methods of improving the crystallinity of the AlGaN based nitride semiconductor are proposed in, for example, Patent Documents 1 to 4. Specifically, Patent Document 1 proposes a method of preventing a crack in AlN from extending upward by forming a buffer layer on a sapphire substrate using a plurality of paired layers each of which is obtained by alternately laminating AlN and GaN. Patent Document 2 proposes a method of reducing a threading dislocation by locally reducing the growth speed of an AlN layer by supplying NH3, which is a material gas of N, in a pulsed manner for a certain period of time at the time of forming a buffer layer composed of AlN on a sapphire substrate. Patent Document 3 proposes a method of reducing a threading dislocation by completely covering island cores composed of AlGaN formed on a sapphire substrate by an AlGaN buffer layer having a larger Al composition ratio than the cores. Patent Document 4 proposes a method of improving the crystallinity of an AlGaN based nitride semiconductor formed above a substrate by optimizing an off angle of the sapphire substrate.